diff --git a/build/libtool.m4 b/build/libtool.m4
index 55465883031..268cad9d3e5 100644
--- a/build/libtool.m4
+++ b/build/libtool.m4
@@ -2871,12 +2871,12 @@ lt_save_path_LD=$lt_cv_path_LD
 if test -n "${lt_cv_prog_gnu_ldcxx+set}"; then
   lt_cv_prog_gnu_ld=$lt_cv_prog_gnu_ldcxx
 else
-  $as_unset lt_cv_prog_gnu_ld
+  unset lt_cv_prog_gnu_ld
 fi
 if test -n "${lt_cv_path_LDCXX+set}"; then
   lt_cv_path_LD=$lt_cv_path_LDCXX
 else
-  $as_unset lt_cv_path_LD
+  unset lt_cv_path_LD
 fi
 test -z "${LDCXX+set}" || LD=$LDCXX
 CC=${CXX-"c++"}